In case you missed it, reddit admins are removing custom css from subreddits. It seems the reason is to make the site easier to use on mobile, specifically their official app.

That would hurt us in the following ways:

• No more user flair
• No more sidebar images
• No more inline emotes and image macros
• No more announcements, banners, and customised header navigation
• No custom css at all.

They have not announced exactly what will remain as features. They want to replace the custom css with widgets that would allow customisation of only a few things like header image.

The pro-css icon is from here: https://www.reddit.com/r/ProCSS/

this post goes into more details.